Hello,
I am trying to make the images appear in grayscale (by using CSS) on the home page, instead of just changing the opacity as it is by default, I would like the images to be in grayscale, then in full color on hover.

I found this part of code in the CSS file, but don’t know how to change it.

You can use the following, but it might not work in all browsers.
#blog img,
#portfolio img{
-webkit-filter: grayscale(100%);
-moz-filter: grayscale(100%);
-o-filter: grayscale(100%);
-ms-filter: grayscale(100%);
filter: grayscale(100%); }